Control of the temperature of reactions within an acceptable range has been widely investigated and the chemical industry has devised several arrangements, those commonly used being discussed in standard references and texts, e.g. one might consider the general teachings by Octave LEVENSPIEL in Chapter 19 of Chemical Reaction Engineering (published by John Wiley & Sons).
The prior art includes a conventional reactor designed to offer more control over the reactant temperature and this is known as the staged adiabatic packed bed reactor. This system uses an arrangement wherein a number of discrete, spaced apart zones of reaction are provided with means therebetween to control the temperature of the products leaving a first zone of reaction prior to entering the next reaction zone. No heat exchanging means is provided to control the temperature of the reaction in the zones of the reaction. Thus the reactant fluid entering the reactor at a desired temperature passes through a packed bed containing catalyst. Upon exiting this first stage, the reactant gas and any products will have a temperature higher or lower than that of the initial temperature depending upon the reaction thermal characteristics. A heat exchanger then heats or cools the reactant gas to a second desired temperature, which may or may not be equivalent to the temperature of the first, before passing to the next packed bed i.e. the second stage. This sequence is repeated until the desired conversion is obtained. Thus the temperature profile of the reaction will be stepped within an acceptable range of temperature, and will therefore not be truly isothermal.
The heat exchanger panel of choice for the purposes of the invention is one formed from a plurality of plates superposed and diffusion bonded to form a stack of plates, wherein fluid passages are defined in said stack by virtue of a pre-treatment of said plates wherein each plate is selectively configured to provide channeled or blank surfaces according to the desired pattern of fluid passages by a treatment to remove surface material e.g. by chemical etching, hydraulic milling, or the like process to a desired depth. Optionally the chemical treatment may be augmented by a mechanical treatment using a suitable tool.
Such a pre-treatment of the plates is conducted in a manner analogous to manufacture of printed circuit boards (PCBs) and for this reason the heat exchanger design can be described as a printed circuit heat exchanger (PCHE). The application of the diffusion bonding technique for metal plates is well understood in the art of metal working and is applied for a variety of purposes e.g. in medical prosthesis manufacture.
This design of the PCHE has been proven by the designers of the proposed PCR system since 1985 when these compact heat exchangers were first introduced.
A PCR type of reactor was designed by the present applicants and is the subject of a separate patent application (GB. 0001699.8). Such a reactor is formed to provide at least one reaction zone, bounded by a heat exchanger formed from a plurality of plates superposed and diffusion bonded to form a stack of plates, wherein fluid channels are defined in said stack by virtue of pre-treatment of said plates wherein each plate is selectively configured according to the desired pattern of channels by a chemical treatment to remove surface material e.g. by chemical etching, to a desired depth. The fluid channels defined in the stack provide the opportunity to arrange for various reactant fluids to be conveyed in channels arranged in heat transfer relationship to discrete channels containing at least one auxiliary fluid for controlling the temperature of the reactants.
In order to maintain adequate control over a reaction, it is preferred that the reactant temperature profile at the exit of the heat exchanger panels is flat, since the reactants pass directly into the following adiabatic bed, without an opportunity to mix on a gross scale. If the reactants are too hot or too cold in places, the selectivity or conversion might well suffer. The issue assumes critical importance in strongly exothermic reactions, as thermal runaway can result if some of the reactants are not sufficiently cooled between reaction stages: higher temperatures result in higher reaction rates, which results in yet higher temperatures.
The heat transfer medium which carries the heat to or from the reactant can be fluids such as water, steam, molten salt, liquid metal, combustion gas or pressurised closed loop gas. When the reaction is near ambient temperature, then the provision of a large flow of heat transfer medium and the supply or extraction of heat from it may present few problems. It may simply be that water is used and the low grade heat either dumped in cooling towers, if the reaction is exothermic, or obtained from exhaust steam, if the reaction is endothermic. Alternatively, boiling or condensing water might be employed at higher temperatures.
However, more of a challenge arises when temperatures are extreme and the heat is more difficult to provide or sink, for example, if water is super-critical and so, depending on the exact situation, boiling and condensing cannot be used for isothermal heat addition or the temperature limits of the materials of construction are being approached or molten salts would degrade.
A typical example is the styrene reaction, which is an endothermic reaction for which the catalyst bed is ideally maintained in the vicinity of 600° C. In the PCR, this might entail allowing the reactant temperature to drop to about 580° C. in each adiabatic bed, with reheat to about 620° C. in each heat exchanger.
A suitable heat transfer medium is superheated steam at a temperature of 750° C. Ideally the outlet temperature of the steam from the heat exchangers would approach the reactant inlet temperature, in order to minimise the flow rate of the steam, and hence minimise the pipe sizes to carry it and the energy losses of its flow.
However, this introduces a severe potential problem: if the heat exchange between the steam, entering at 750° C. and cooling towards the reactant inlet temperature, and the reactant, entering at 580° C., is not carefully managed then the temperature of the reactants exiting the heat exchanger will vary widely, even if the average reactant exit temperature is the required 620° C.
The problem is illustrated in FIG. 1 where, when there is a simple cross flow contact between the steam and the reactants it is noticeable that the higher reactant exit temperature is biased toward the end of the plate on which the steam enters. Furthermore, although an average temperature of 620° C. is produced, the variation in reactant exit temperature across the width of the heat exchange panel is approximately 580° C. to 720° C., a span of +/−70° C. This would be detrimental to the yield of the reaction.
An object of the invention is to obviate or mitigate problems of this nature. In particular it is an object of this invention to provide apparatus including a chemical reaction zone and a method of controlling the temperature of reactants to be reacted in such a zone.